A common metal cutting task, especially in the automotive industry, is the milling of flat, smooth surfaces on metal workpieces that are initially rough, such as cast iron engine blocks and heads. Planning a milling operation involves both the design of the milling cutter, and deciding upon the parameters at which to operate it. The current invention concerns basically the design of the milling cutter. However, the background of both milling cutter design and the parameters of milling operation will be discussed in order, since the two are related.
The subject invention concerns a type of milling cutter known as an "indexable" milling cutter, so called because it uses replaceable, or indexable cutting inserts. The cutting inserts are manufactured in standard sizes from a hard material, such as tungsten carbide (coated or uncoated), silicon nitride or diamond, and may be discarded and replaced when dulled. A typical example of a commercially available indexable insert is the one indicated generally in FIG. 8 at 10. Insert 10 is uncoated tungsten carbide, and has the general shape of a symmetrical rectangular wafer with eight cutting corners, and has a width of 0.375 inches, a length of 0.625 inches, and a thickness of 0.1875 inches. The shape of insert 10 identifies it as what is known in the art as a roughing insert, so called because it is generally used to remove an initial layer of material in the milling process. One of the eight cutting corners of insert 10, which are actually short 45 degree chamfers or flats of approximately 0.015 inches, is indicated at 12. These short chamfered corners 12 are treated as the equivalent of points, for purposes of measuring their geometry and spatial location, which will be further described below. The wider flat portions of insert 10 may be referred to as top and bottom faces 14, and the narrower flat portions may be referred to as side faces 16. Another common type of insert is known as a wiping insert, a typical example of which is designated at 20. Wiping insert 20 is of generally the same shape as 10, with the same width, length and thickness, and also has eight cutting corners, one of which is indicated at 22. The cutting corners 22 of a wiping insert 20 are generally cut at a shallower angle, fifteen degrees here, which gives the corner 22 a longer surface, approximately 0.060 inches. A wiping insert is used to remove a thinner layer of material in a final smoothing operation, and engages the workpiece last. It is known to mount the wiping inserts to either the same or a different milling cutter body as the roughing inserts, although it is generally convenient to use the same cutter body.
Designing an indexable milling cutter consists basically of deciding on a geometry or configuration for the inserts, in terms of radius, axial height, and angular orientation. The payoff for a good design is that, for any given operating parameters and insert material, insert wear will be reduced. Once the geometry is determined, the inserts are conventionally mounted by central threaded fasteners into locating slots cut into a generally cylindrical and rotatable metal cutter body, of which the cutter body 18 of the subject invention, FIG. 5, is a good example. So mounted, each insert presents one cutting corner at a time to the workpiece, which may be referred to as the working cutting corner, with the other seven corners held in reserve. After the working cutting corner has worn with normal use, the insert may be easily removed, turned and remounted to present a new cutting corner to the workpiece. This is known as "indexing" the insert. An insert is indexed until all eight cutting corners are worn, after which it is discarded.
Considering the insert pattern generally, they are mounted in a circular pattern or patterns about the cutter body centerline axis, with the working cutting corners located at a predetermined radius. A basic requirement for the overall size of the insert pattern is that the inserts at the smallest radius must be at least widely enough spaced to cover the entire surface that is to be milled. This is generally stated as a rule that the effective diameter of the cutter must be at least as wide as the milled surface. In fact, it is generally recommended that the effective diameter should be at least 1.6 times width of cut, so that the cutter can be positioned to overhang the surface to be milled by a fourth to a third. The exigencies of the workpiece and the environment may dictate a lesser effective diameter relative to the width of cut, however.
As far as the specific positioning of individual inserts on the cutter body, the cutting corners of inserts of the same type, that is, roughing or wiping, are generally located at the same radius, and also lie in the same plane. This is because the basic theory of operation has been that each insert operates independently of, but in the same way as, every other insert of the same type. Each insert will also generally have a predetermined radial and axial rake angle, terms that will be well known to those skilled in the art. The rake angles of an insert are measured between a reference plane and a reference face of the insert. The reference plane is the plane that passes through the cutter body centerline axis and the insert's working cutting corner (which is treated as a sharp corner, even if it is a short chamfer). The insert reference face is the face that sees the workpiece, which, of course, is dependent upon the direction of cutter rotation. The insert's radial rake angle is the angle formed by the reference plane and the reference face as measured in a plane perpendicular to the cutter body axis. The insert's axial rake angle is the angle formed by the reference plane and the reference face measured in a plane perpendicular to the radius of the cutter body, at the working cutting corner. A rake angle, radial or axial, is considered positive if the insert reference face, that is, the insert face forming the angle, slopes away from the direction of cutter rotation, and negative if it slopes toward the direction of rotation. Each insert may also have what is referred to as a bevel or lead angle, which is the angle formed between the radially outwardly facing insert edge that includes the working cutting corner and a line parallel to the cutter body axis that passes through the cutting corner. Specifying an axial and radial position for the working cutting corner of each insert, as well as an axial and radial rake angle and bevel angle, serves to absolutely establish each insert's position on the cutter body. Each insert's position relative to the other inserts, which is related to the total number or density of inserts, must also be established. The rule of thumb is that the greatest number of inserts possible, given the room available on the cutter body, should be used.
Operational parameters to be chosen include depth of cut, cutting speed, and linear feed rate. The total metal removal rate is proportional to all three, so it is generally desirable to maximize all three. However, since it is also desirable to minimize tool wear, and since each of the three parameters has a different effect on tool wear, there are trade offs. The one factor that is set in stone for the tool designer is the workpiece. It will have a certain roughness after it leaves the mold, a certain hardness, and the cut to be made will have a certain width. The initial roughness out of the mold will dictate what depth of cut is necessary to work the rough surface down to a suitably flat and smooth final surface. And, since insert wear generally increases with an increase in depth of cut less than it does with an equivalent percentage increase in cutting speed, it is generally recommended to maximize the depth of cut. The cutting speed of any insert, that is, the surface speed in feet per minute of the insert relative to the workpiece surface at the cutting interface, depends on its radius and on the rotational speed of the cutter body. The cutter body is rotated by a milling machine, such as the conventional milling machine 24 in FIG. 1. Those skilled in the art will have a general idea, for any given workpiece material and insert material, what cutting speed to use, knowing what has worked in the past. The possible cutting speed is limited by the relative hardnesses of workpiece and insert material. The art teaches that cutting speed has a large negative effect on insert life as it is increased. Clearly, the harder the insert relative to a given workpiece material, the higher the potential cutting speed. However, the cost of cutting insert materials goes up more than proportionally to hardness, with silicon nitride, for example, being significantly more costly than tungsten carbide and diamond being more expensive yet. The designer will naturally desire to use the least costly, softest possible insert material. The rotating cutter body 18 is also driven linearly by the milling machine 24 across the workpiece at a desired feed rate. Once a cutting speed is chosen, a feed rate is determined. This may be determined by choosing a feed per insert (sometimes expressed as a feed per blade), and multiplying it by the number of inserts. Since insert wear does not go up as steeply with increasing feed rate as it does with increasing cutting speed, it is also generally recommended that feed per insert be pushed as high as possible. As a practical matter, an operator, given a depth of cut, cutting speed, and insert material, would probably test the feed rate by pushing it progressively higher, and stop at the highest feed rate consistent with economically acceptable tool wear and surface quality.
What is economically acceptable is subjective, of course, but the past experience of the inventors of the subject invention provides perhaps the best evidence of what was considered possible prior to the invention. Trade literature on speeds and feeds is sometimes difficult to compare, because depths of cuts and surface qualities may not be specified. With the subject invention, however, only the milling cutter design was changed, with all other factors, like the milling machine, workpiece and operating parameters, remaining substantially the same. In general, prior to the subject invention, an economically unsatisfactory choice for insert material, and a very costly and high rate of insert wear, were tolerated in order to achieve acceptable productivity and surface quality. A typical workpiece was the cast iron engine head 26 of FIG. 1, which is grey cast iron with a Brinnell hardness of 187 to 255, and a total carbon percentage by weight of 3.00-3.50. The milling cutter previously used was of the type known as a slash mill, indicated generally at 28 in FIG. 2. Slash mill 28 has a cutter body 30 to which ten inserts 32 are mounted, all at the same radius, axial depth and angles, as well as evenly circumferentially spaced. Slash mill 28 was run at an RPM of approximately 3,000 to 3,500, and at a feed rate of approximately 240 inches per minute. Even with an insert 32 of diamond, the hardest and most expensive material, the outside limit of the surface quality specification was soon reached, which was easily detectable as a roughening or furrowing of the milled surface. The outside limit would be reached sometimes after as few as a thousand parts per working cutting corner. On milled surfaces with sharp edges, like the surface between the cylinder bores of the engine block 34 of FIG. 3, edge chipping or breakout 36 would begin to occur at an early point, also necessitating insert indexing. It is now thought that these problems were caused by the size of the metal chips produced by the slash mill 28, indicated at 38 in FIG. 4. These measure on average about 0.006 inches in thickness, and 0.120 inches in width. At the time, however, no cutter tried had given a significantly smaller chip. While a smaller chip might have resulted in a better surface quality, at least, there was no indication in the art that smaller chips would have resulted in improved insert life and reduced insert wear at the high cutting speeds involved. If anything, the skill in the art indicated that minimizing feed per insert, which would logically lead to smaller chips, would decrease productivity.